Orchids and Their Pollinators

By David Horak | April 1, 2004

Some flowering plants are promiscuous, relying on enticement and reward to
lure whatever insect comes along. But orchids typically have exclusive
relationships with their pollinators. These are usually bees, wasps, and flies,
but many orchids also utilize moths, butterflies, fungus gnats, or birds to
cross-pollinate their flowers. While the ways that various species entice
pollinators to visit their flowers and carry their pollinia (pollen masses) off
to a flower on another plant vary tremendously, they often employ complex,
frequently deceptive strategies to achieve success.

Bloom Time

Governed by the weather and pollinator activity, orchids regulate their
bloom time to increase the likelihood of successful pollination. The length of
time that flowers remain open and viable varies considerably from a few hours
in Flickingeria species to as much as nine months in species such as
Dendrobium cuthbertsonii.

Some orchids flower only once or twice a year at very specific times. Others
have flowers opening so frequently during the course of the year that they are
almost constantly in bloom. Orchids that bloom frequently or for extended
periods certainly have less difficulty coordinating blooming with others of
their kind, increasing the chances of successful cross-pollination. By
contrast, species with short bloom periods would seem to be at a severe
disadvantage. Orchids in the New World genus Sobralia, whose flowers
typically are open for no more than one day, solve this problem by relying on a
group temperature signal to achieve simultaneous bloom. Several days after a
trigger temperature is reached, all the individual plants in a given population
flower at the same time.

Lures and Rewards

Orchids utilize a variety of intricate strategies for attracting the many
specific pollinators on which their survival depends. In virtually all cases
the orchid and pollinator have evolved together.

Microtis parviflora lures its ant pollinator with a reward of nectar.

Prasophyllum fimbria uses deception to attract its pollinators: It seems to offer a food reward but doesn't.

Orchids commonly use nectar to entice their pollinators, but they also
employ color, shape, or fragrance, and even mimic the flowers of other plants.
Color is often important for animals active during the day. For example,
orchids pollinated by hummingbirds and butterflies tend to have red, orange, or
pink tubular—but not necessarily fragrant—flowers. They frequently
have yellow blotchy patterns to mimic the anthers and pollen of the other plant
types visited by these nectar lovers, but this mimicry is deceptive because the
orchids often offer no nectar reward. The flowers of most moth-pollinated
species, such as angraecoids, are green or white and are often very fragrant
only at night so they can be found by these primarily night-flying insects. The
flowers of orchids pollinated by flies or carrion beetles, such as many of the
Bulbophyllum species, typically come in browns and fleshy reds and emit
the odor of rotting meat. The stench of Bulbophyllum beccarii is so foul
that it has been said to smell like "a herd of dead elephants."

A male wasp pollinates Chiloglottis reflexa while attempting to copulate with the flower.

Little iridescent euglossine bees can be seen buzzing in the canopies of
Central American forests when the bucket orchids (Coryanthes) are in
bloom. As the name suggests, in these species the lip of the flower forms a
bucket filled with a sweet-scented viscous liquid. Just above, they also have a
rounded pad from which the male bees scrape off fragrant oils they pack into
sacks on their back legs and use to court females. Actually elaborate traps,
the surface of the flower is slippery, and occasionally a bee loses his footing
and falls into the bucket. The only way to escape and avoid drowning is through
a narrow opening at the base of the lip. As the bee squeezes his way through,
his back scrapes against the column and the two pollinia are deposited on his
back. Fortunately for the orchids, the bees seem to have short memories and are
easily fooled: After drying himself and flying off, the bee will often visit
another flower and repeat the process, this time depositing the pollinia on the
stigma (female part) of the flower.

Some orchids lure their pollinators through sexual deception. Oncidium
henekenii is pollinated by male bees trying to mate with the flower, which resembles a female bee.

Some orchid flowers deceive their pollinators by mimicking the appearance
and scents produced by female insects. In European species of the genus
Ophrys, flowers have a labellum that looks like the body of a fetching
female bee or wasp—complete with the requisite shape, iridescent colors or
colorful markings, and hairs. They also exude a scent that simulates the
pheromones produced by receptive females. Each species of Ophrys is
generally pollinated by its own species of insect. When the male lands on the
flower, it grabs the labellum and attempts to copulate with it. In the process,
the flower deposits pollinia on the insect's head, to be carried and placed on
the next flower he visits. Other orchids, such as species of Oncidium,
produce flowers that resemble the males of certain territorial bees or wasps.
Seen as competitors, these flowers are attacked. They are shaped in such a way
that the attacking insect is inevitably placed in contact with the pollinia or
stigma.

Secure Attachments

Where the pollinia become physically attached to the pollinator and how they
get there is individual to each given orchid species. One example involves
euglossine bees, common shared pollinators for orchids in Central and South
America. The structural differences in the various flowers ensure that the
pollinia are attached to a part of the bee specific to each orchid species: The
pollinia of one may be attached to the insect's eye, that of another to the top
of the thorax, and that of still another to a foreleg. When the pollinia-loaded
bee encounters an orchid flower, only the pollinia in the proper position for
that species will come in contact with the stigma and accomplish
pollination.

In the genus Catasetum, the pollinia are ejected when the pollinator
touches a specifically placed trigger and are literally shot onto the insect's
back. Some species of Bulbophyllum and Porroglossum have hinged
lips that snap shut or tip closed, temporarily pinning the insect against the
column so that the pollinia can be properly secured.

Orchid Seeds

In the wild, the chance of successful pollination varies tremendously among
orchid species. In some, nearly every fertile flower is pollinated; in others,
few or none are. Once pollinated, the flower begins to collapse and the ovary,
located directly behind the sepals, begins to swell. The time it takes for
orchid seed to mature varies from a few days to nearly a year. When finally
mature, the fruit, a capsule, splits open and the seed spills out.

Orchids produce enormous amounts of minute seeds that are disbursed widely
by the wind.

Most orchid species have very specific needs regarding where they are able
to germinate and grow. To ensure that seeds find these ideal conditions,
orchids produce vast amounts of minute seeds, which are disbursed widely by the
wind. Harvard University's Oakes Ames, a prominent early-20th-century orchid
researcher, reportedly spent a rainy afternoon counting the seeds in an
individual orchid capsule and arrived at the astonishing total of 3.5
million.

Because orchid seeds are so small, however, they contain virtually no
endosperm, the food reserve on which young plantlets typically depend prior to
the development of roots and leaves. To germinate in the wild, tiny orchid
seeds must become infected by a mycorrhizal fungus that produces substances
necessary for germination and growth.

Of the very few orchid seeds that are able to find ideal conditions to
germinate, few survive to maturity. It generally takes most wild orchids five
to seven years to reach blooming size. Mortality rates are high during the
fragile early stages of orchid growth.

Early efforts to artificially raise orchids from seed in the 19th century
were unsuccessful until growers discovered that seed sowed at the base of the
mother plant would sometimes germinate. This was possible because the parents
were still naturally infected with the necessary mycorrhizal fungi. However,
even this technique resulted in relatively few plants, so the commercial orchid
industry continued to depend on vast quantities of wild-collected plants to
satisfy the constant demand of wealthy collectors.

In 1922, Louis Knudson discovered that nearly 100 percent germination could
be achieved by starting seeds in flasks on sterile media fortified with
nutrients to feed the young plants. Some years later the process of mericloning
was developed, allowing the mass production and marketing of individual
cultivars. Thanks to these achievements, combined with humanity's insatiable
desire to create artificial hybrids—presently more than 100,000 have been
registered—today orchids are produced by the millions. They are now among
the most widely grown and popular flowering pot plants in the world.

For a comprehensive discussion of orchid ecology, consult the monumental
work The Orchids: Natural History and Classification (1981, 1990), by
Robert L. Dressler.

Anatomy of an Orchid Flower

Despite the astonishing diversity found in the thousands of wild species and
man-made hybrids, orchid flowers show an unmistakable family resemblance.
Orchid flowers are generally bilaterally symmetrical—only a single
imaginary line can be drawn through a flower to create a mirror image.

A typical orchid flower has three sepals (the outer segments that protect
the bud before the flower opens), alternating with three petals. The petals and
sepals may be similar or not, showy or inconspicuous.

The middle petal, which is always opposite the column, is usually quite
different from the others. Called the lip or labellum, it comes in a variety of
shapes, depending on the species, and can be wavy or fringed or covered with
hairs or other structures. The labellum often serves as a landing pad and
attractant for insect pollinators.

The most distinctive aspect of orchid floral anatomy is the column, the
single reproductive structure formed by the fusion of the male stamens and
female style, which are separated in the vast majority of plant families. Most
orchids have a single fertile anther (flower structure where pollen is
produced) located at the tip of the column.

In most orchids, pollen is not loose and granular when ripe but rather is
packed into a waxy mass called a pollinium. Pollinia usually occur in pairs,
but in some species they are found in groups of up to eight. The pollinia
typically share a single small sticky tab called a viscidium, which adheres to
the pollinator when contact is made.

To discourage self-pollination and promote cross-pollination, the pollinia
typically are separated from the stigma (female part of the flower that
receives the pollen) by a flap of tissue called the rostellum. The rostellum
also aids in the transfer of the pollinia from the pollinator to the
stigma.

There are exceptions to the typical orchid flower configuration, shown
above. For example, slipper orchids such as paphiopedilums, considered a
primitive branch of the orchid family, have a pouch- or slipper-shaped
labellum, below left. They also have two fertile anthers instead of a single
anther, below right. A third anther has evolved into a fleshy plate, called a
staminode, which sits in front of the other two and assists in luring and
guiding pollinators across the anthers and stigma.

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Discussion

I was searching on the web to understand what pollinates orchids (Wikipedia was not very helpful!), and was surprised to find such a detailed and well-written article here. Thank you for this!

Sandra
June 25, 2017

I have a large Cattlelya orchid planted in coconut fiber with bark, and I have noticed a a family of wasps burrowing in from the bottom of my orchid. Are they pests or pollinators? It hangs outside.

BBG Staff
October 28, 2016

Hi, Lugano: Your topic sounds interesting and also extensive. Without knowing the specifics of your project, including what research you have done already, it is difficult for us to send you articles pertaining to your research. That said, check out this short article from Smithsonian Magazine.

Lugano Dula
September 5, 2016

I am a student of the University of Dar es Salaam, studying bachelor of beekeeping science and technology…I become interested with this article because it’s related to my research about bee flowers and behavior…What orchid flowers do bees like to forage?

Kim Feddersen
June 29, 2014

I find it hard to believe that 3.5 million pollen have to grow tubes through the stigma to produce a seedpod with that number of seeds. Is orchid seed production based on on one pollen for each ovule? Also, seed developmental times of 6 to 18 months seem very long compared with most other plants. Is there some additional mechanism in the orchid seed production process?

francisco loaiza
July 21, 2010

I want learn very much of the orchids

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